The production from a subsea well is controlled by a number of valves that are assembled into a Christmas tree. The actuation of the valves is normally dependent upon hydraulic fluid to operate hydraulic actuators for the valves and is therefore entirely dependent upon an external source for the supply of the hydraulic fluid. Hydraulic fluid is normally supplied through an umbilical running from a station located on a vessel on the surface or, less commonly, from a land based station. Usually the actuators are controlled by pilot valves housed in a control module located at or near the subsea installation. The pilot valves direct the supply of fluid to each actuator, as dictated by the need for operation. The pilot valves may be operated by electric means, and such a system is therefore called an electrohydraulic system.
The design of actuators and valves for subsea wells is dictated by stringent demands on the standard and function for these valves because of the dangers of uncontrolled release of hydrocarbons. A typical demand is that these valves must be failsafe closed, meaning that they must close upon loss of power or control. The only practical means today in subsea environments is to use springs that are held in the compressed state by the hydraulic pressure, keeping the valve open, and are released in the event of loss of hydraulic pressure, thus closing the valve. The spring force needed to close a valve is dependent on both the well pressure and the ambient pressure, with larger ambient pressure demanding larger springs.
For the control of subsea wells, a connection between the well and a monitoring and control station must be established. This station can either be located in a floating vessel near the subsea installation or in a land station a long distance away. Communication between the control station and the subsea installation is normally provided by installing an umbilical between the two points. The umbilical contains lines for the supply of hydraulic fluid to the various actuators in or by the well, electric lines for the supply of electric power and signals to various monitoring and control devices, and lines for signals to pass to and from the well. This umbilical is a very complicated and expensive item, costing several thousand dollars per meter.
It would therefore be very cost-saving to be able to eliminate the umbilical. In the invention, the standard hydraulic lines can be eliminated while maintaining the standard hydraulic spring-operated failsafe system.
European Patent application No. 1209394 discloses an electro-hydraulic control unit with a piston/cylinder arrangement in which the piston divides the cylinder into two chambers, a fluid connection between the two chambers, and a valve to configure the fluid flow such that pressurized hydraulic fluid may only flow in one direction and not in two directions.
U.S. Pat. No. 6,269,874 discloses an electro-hydraulic surface controlled subsurface safety valve actuator that comprises an electrically actuated pressure pump and a dump valve that is normally open so that if power fails, the pressure is released and the safety valve closes.
An electro-hydraulic actuator similar to the present invention is also known that comprises a pump unit that supplies hydraulic fluid under pressure to a piston so as to move the valve to its open position. The actuator is intended to be fitted to an existing valve. In this disclosed actuator, the failsafe function is provided by a two-way valve that is located in a bypass fluid line connecting one side of the piston with a drain accumulator. The two-way valve is fitted with a solenoid that closes the bypass line and a spring that biases the two-way valve towards a position where fluid can flow through the bypass line. Upon loss of power to the solenoid, the spring will force the two-way valve to open the bypass passage. The solenoid must therefore be strong enough to withstand the force of the spring. Also, the spring must be strong enough to force the valve open against the pressure in the line. This pressure can be very high, especially in systems located at great depths.
The object of the present invention is to provide a smaller and more compact actuator that can be releasably fitted to the valve and which overcomes the disadvantages related to the prior art.
A further objective of the invention is to provide an actuator that overcomes the shortcomings identified above.
The invention is set forth in the independent claim 1, while the dependent claims define other characteristics of the invention.